High power lateral MOSFETs typically have a relatively low dopant concentration drift region, of the same conductivity type as the drain, leading from the drain to the channel region. When the MOSFET is off, the drift region depletes somewhat to achieve a desired breakdown voltage. Higher breakdown voltages are achieved with longer drift regions, given a certain dopant concentration in the drift region. The low doping level of the drift regions adds significant on-resistance. Further, when the MOSFET is off, the electric field is concentrated near the PN junction at the surface of the drift region under the gate, which limits the breakdown voltage.
A RESURF (Reduced Surface Field) structure is sometimes used to increase the breakdown voltage of a MOSFET and/or reduce the on-resistance and replaces the conventional drift region with alternating thin layers of P and N-type material. When the MOSFET is off, the P and N-type layers become entirely depleted more evenly along their length so the electric field is not concentrated near the channel region. This behavior results in a higher breakdown voltage and enables higher doping levels to be used in the RESURF layers which reduce on-resistance.
FIG. 1 is a reproduction of a figure from U.S. Pat. No. 6,097,063, showing a cross-sectional view of a RESURF structure in a lateral MOSFET with a deep trenched gate. To turn the MOSFET on, a sufficiently positive voltage is applied to the trenched gate 111 to invert a deep P-body region (not shown in the cross-section but vertically adjacent the gate 111) to conduct current between an N+ source (not shown) and the N+ drain 99. Layer 10 is a gate oxide. The alternating P and N layers 1, 2, and 2a are the RESURF layers 100, and the N-layers 1 conduct the current. The substrate 4 can be either N- or P-type. The top layer 12 is a dielectric film.
Forming deep P-body/channel regions and trenched gates adds to the structure complexity and results in a MOSFET with a threshold voltage that is difficult to control. Furthermore, connecting the P layers in the RESURF structure at the source side to the source electrode (via the P-body region) without limiting the current flow is difficult.
What is needed is an improved device structure for a RESURF lateral MOSFET, where a relatively shallow P-body channel region is inverted using a gate located close to the surface and where the P layers in the RESURF structure are easily connected to the P-body region.
What is also desirable is an improved design for a RESURF structure in a lateral MOSFET that exhibits an effectively long drift region (for high breakdown voltage) without directly increasing the top surface area of the RESURF structure.